1. Field of the Invention
This invention relates to a catalyst and process for the conversion of hydrocarbons, and more specifically for the catalytic reforming of gasoline-range hydrocarbons.
2. General Background
The catalytic reforming of hydrocarbon feedstocks in the gasoline range is an important commercial process, practiced in nearly every significant petroleum refinery in the world to produce aromatic intermediates for the petrochemical industry or gasoline components with high resistance to engine knock. Demand for aromatics is growing more rapidly than the supply of feedstocks for aromatics production. Moreover, the widespread removal of lead antiknock additive from gasoline, reformulation of gasoline for reduced emissions, and the rising demands of high-performance internal-combustion engines are increasing the required knock resistance of the gasoline component as measured by gasoline "octane" number. The catalytic reforming unit therefore must operate more efficiently at higher severity in order to meet these increasing aromatics and gasoline-octane needs. This trend creates a need for more effective reforming processes and catalysts.
Catalytic reforming generally is applied to a feedstock rich in paraffinic and naphthenic hydrocarbons and is effected through diverse reactions: dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins, isomerization of paraffins and naphthenes, dealkylation of alkylaromatics, hydrocracking of paraffins to light hydrocarbons, and formation of coke which is deposited on the catalyst. Increased aromatics and gasoline-octane needs have turned attention to the paraffin-dehydrocyclization reaction, which is less favored thermodynamically and kinetically in bifunctional reforming than other aromatization reactions. Considerable leverage exists for increasing desired product yields from catalytic reforming by promoting the dehydrocyclization reaction over the competing hydrocracking reaction while minimizing the formation of coke.
The effectiveness of reforming catalysts comprising a non-acidic L-zeolite and a platinum-group metal for dehydrocyclization of paraffins is well known in the art. The use of these reforming catalysts to produce aromatics from paraffinic raffinates as well as naphthas has been disclosed. The increased sensitivity of these selective catalysts to sulfur in the feed also is known. Commercialization has been slow in coming and is limited in scope in light of the special measures that must be taken to exclude sulfur from the process. There is a need for a catalysts demonstrating enhanced stability as well as improvements in the high selectivity featured by these dehydrocyclization catalysts.
The art discloses reforming with a broad range of catalysts containing large-pore zeolites and Group VIII metals. U.S. Pat. No. 4,104,320 (Bemard et al.) discloses dehydrocyclization with potassium-form L-zeolite charged with one or more dehydrogenating metals of Group VIII, but teaches that two metals would be introduced simultaneously and does not suggest the advantages of controlling the distribution of is metal in the catalyst. U.S. Pat. No. 4,914,068 (Cross et al.) teaches a method of dispersing at least one Group VIII metal in the pores of a large pore zeolite, along with a specified amount of a non-platinum metal salt.
Non-uniform distribution of metal within a catalyst has been disclosed for some catalysts. U.S. Pat. No. 3,259,589 (Michalko) discloses a variety of catalyst physical structures characterized by the placement of a layer of a metal component, but does not suggest that the metal component should comprise metals with different gradients. U.S. Pat. No. 4,677,094 (Moser et al.) teaches a trimetallic catalyst comprising uniform platinum and tin and a surface-impregnated metal selected from one or more of rhodium, ruthenium, cobalt, nickel, and iridium, and teaches that the surface-impregnated component is incorporated into the catalyst support of an acidic catalyst. U.S. Pat. No. 4,786,625 (Imai et al.) teaches a catalytic composite comprising surface-impregnated platinum group metal and uniform tin, germanium, or rhenium on a refractory oxide support. None of the above references anticipate or suggest a catalyst containing a nonacidic large-pore molecular sieve, a platinum-group metal component, and a surface-layer promoter metal.